<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <!--兄弟节点，有相同的父节点-->
    <!--父子节点，子节点直接嵌套于父节点中-->
    <div id="con">
        <div class="first">
            <div class="third"></div>
        </div>
        <div class="second">
            <div class="fourth"></div>
        </div>
    </div>
</body>
</html>